1. Field of the Invention
The present invention relates to a color transformation method and a recording medium with a color transformation program recorded thereon, and more particularly to a color transformation method for transforming device data in a first device space represented by a first color system into device data in a second device space represented by a second color system different from the first color system, as well as to the recording medium with a color transformation program recorded thereon.
2. Description of the Related Art
In recent years, in conjunction with the consolidation of communication facilities of open networks, in devices such as image displaying devices and printing devices connected to the networks, images are presented by using image source data including color information of other devices. In this case, it is necessary to transform image source data (hereafter referred to as the device data) including color information of the devices between color spaces (hereafter referred to as the device spaces) of the devices. For example, in a case where an image is color printed by using RGB data which is mainly used in image displaying devices, the RGB data must be transformed into CMYK data which is mainly used in the printing devices.
Conventionally, when transforming RGB data to CMYK data, the device spaces of RGB and CMK are temporarily mapped on a generally used color space (hereafter referred to as the colorimetric space) such as an L*a*b* color space or an XYZ color space. Then, a transformation from the RGB device space into the colorimetric space and a transformation from the colorimetric space into the CMYK device space are sequentially effected. Namely, it has been general practice to transform the RGB data into color information in the colorimetric space, and then the color information in the colorimetric space into the CMYK data.
However, the reproducible range, (also known as the gamut), in the colorimetric space, which corresponds to each device space, differs among the respective devices. FIG. 13 shows a conceptual diagram illustrating the configurations and ranges of a gamut 90 of a device space (hereafter called the RGB device space) based on the RGB color system in the colorimetric space and a gamut 92 of a device space (hereafter called the RGB device space) based on the CMYK color system. As shown in FIG. 13, the gamut 90 and the gamut 92 differ from each other.
In a case where data conversion which temporarily undergoes the above-described mapping on the colorimetric space is effected between the RGB device space and the CMYK device space having different gamuts in the colorimetric space, in a forward direction (RGB device space ∃ colorimetric space), the gamut 90 in the RGB device space must be enlarged to a region (a region including both the gamut 90 and the gamut 92) which includes a region 94 which is unnecessary as an actual gamut, whereas, in a reverse direction (colorimetric space ∃ CMYK device space), mapping must be effected on the CMYK device space by including a region 96 which is not required as a gamut in the CMYK device space.
For this reason, it is general practice to subject data corresponding to the gamut 90 and the gamut 92 in FIG. 13 to processing such as space compression, expansion, truncation, and the like, and there is a possibility that reproduction accuracy declines in the case of color reproduced from data subjected to processing such as space compression, expansion, truncation, and the like. In addition, since it is difficult to uniformly effect mapping in the entire gamut of the destination CMYK device space to which the transformation is effected to, there is a possibility that maximum use cannot be made of the gamut of the CMYK device space (i.e., an effective gamut is lost).
In addition, since the device space and the L*a*b* color space do not exhibit complete agreement in discrete sampling characteristics and quantization characteristics, if the conversion which temporarily undergoes mapping on the L*a*b* color space is effected, discontinuous CMYK data is obtained with respect to the RGB data which continues at predetermined intervals, so that there is a possibility that the quality of the image which is printed out deteriorates.
Further, K data has a characteristic that it is equivalently replaced by a gray component which is represented by a predetermined amount of equivalent CMY data, and innumerable CMYK data which are equivalent to one arbitrary piece of CMYK data are present, as shown in FIG. 14. Accordingly, when RGB data is converted to CMYK data, since innumerable combinations of CMYK device space can be obtained, there arises the need to set some restricting conditions on the conversion.
Further in actual printing, the reproduction of color and density must be ensured. In terms of physical properties of printing, the color reproduction characteristic does not necessarily follow an increase in the amount of ink linearly. Namely, the additive process with respect to the amount of ink does not hold, and an unstable gamut is unfailingly present. This is attributable to the fact that there is an upper limit to the ink accepting capacity of printing ink which is a reproducing medium. Therefore, in converting RGB data to CMYK data, it is desirable to develop the gamut by avoiding the aforementioned unstable gamut.
When the RGB device space represented by a monitor or the like is mapped on the CMYK device space peculiar to printing, it is difficult to isochromatically convert all the colors owing to their difference in the gamuts. Accordingly, as general color matching means, various methods shown below in Table 1 are adopted depending on purposes, each having their merits and demerits.
In the perceptual method, all the colors are relatively converted within the gamut, but retention of isochroism with respect to data within the gamut and outside the gamut is not ensured, and in a case where the gamuts of the device space before and after conversion are substantially different, there is a possibility of the atmosphere of the image becoming entirely different due to the conversion. In addition, with respect to the remaining three methods in which measures are devised for mapping for only the region outside the gamut, in the case of the absolute colorimetric method, if the gamuts differ substantially, there is a high possibility that data corresponding to the region outside the gamut is truncated, while in the relative colorimetric system and the saturation method, the continuity of gradation in the target device space is not ensured.
The present invention has been devised to overcome the above-described problems, and it is an object of the present invention to provide a color transformation method capable of effecting transformation of the device data between different device spaces.
To this end, in accordance with a first aspect of the present invention, there is provided a color transformation method for transforming device data in a first device space represented by a first color system into device data in a second device space represented by a second color system different from the first color system, comprising the steps of: determining a first relationship of correspondence from the device data in the first device space to color information in a third color system different from the first and the second color systems and a second relationship of correspondence from the device data in the second device space to color information of the third color system; determining an inverse relationship of correspondence from the color information of the third color system to the device data in the second device space on the basis of the second relationship of correspondence; determining a forward relationship of correspondence from the device data in the first device space to the device data in the second device space on the basis of the first relationship of correspondence and the inverse relationship of correspondence; and transforming the device data in the first device space into device data in the second device space by using the forward relationship of correspondence determined.
In addition, in accordance with a second aspect of the present invention, there is provided a color transformation method for transforming device data in a first device space represented by an RGB color system into device data in a second device space represented by a CMYK color system different from the RGB color system, comprising the steps of: determining a first relationship of correspondence from the device data in the first device space to color information in a third color system different from the RGB color system and the CMYK color system and a second relationship of correspondence from the device data in the second device space to color information of the third color system; determining an inverse relationship of correspondence from the color information of the third color system to the device data in the second device space on the basis of the second relationship of correspondence after K data of the device data in the second device space is constrained under a predetermined condition; determining a forward relationship of correspondence from the device data in the first device space to the device data in the second device space on the basis of the first relationship of correspondence and the inverse relationship of correspondence; and transforming the device data in the first device space into device data in the second device space by using the forward relationship of correspondence determined.
In addition, in the color transformation method according to the second aspect of the invention, the predetermined condition is a condition that the lower the lightness and saturation of a color corresponding to the color information of the third color system, the more the K data is increased.
The color transformation method in accordance with the above-described first aspect of the invention is a color transformation method for transforming device data in a first device space represented by a first color system into device data in a second device space represented by a second color system different from the first color system. In this color transformation method, a first relationship of correspondence from the device data in the first device space to color information in a third color system different from the first and the second color systems and a second relationship of correspondence from the device data in the second device space to color information of the third color system are first determined.
It should be noted that the first and second relationships of correspondence referred to herein may be functions or lookup tables, for example. As the aforementioned device spaces, it is possible to cite a device space represented by the RGB color system and a device space by the CMYK color system. As the third color system, it is possible to adopt a generally used color space (colorimetric space) such as the L*a*b* color space and the XYZ color space.
In addition, this color transformation method is applicable to a method in which the RGB color system is adopted as the first color system, the CMYK color system is adopted as the second color system, and transformation is effected from RGB data into CMYK data. Conversely, this color transformation method is applicable to a method in which the CMYK color system is adopted as the first color system, the RGB color system is adopted as the second color system, and transformation is effected from CMYK data into RGB data. It goes without saying that the aforementioned CMYK data includes CMYK data (CMY data) in which K data in the CMYK color system is fixed to a predetermined value (e.g., xe2x80x9c0xe2x80x9d).
Next, as an inverse solution of the second relationship of correspondence, an inverse relationship of correspondence from the color information in the third color system to the device data in the second device space is determined. For example, in a case where the relationship can be expressed by a square matrix as in the case of the relationship of correspondence between an sRGB space (which is a standard RGB device space which is univalently made to correspond to the colorimetric space) and the L*a*b* space, the inverse relationship of correspondence is determined simply as an inverse matrix. However, in a case where the relationship of correspondence with the colorimetric space cannot be expressed by a simple matrix as in the case of the CMY device space for printing, the inverse relationship of correspondence is determined by successively obtaining inverse solutions from the second relationship of correspondence.
Here, by applying the device data in the first device space to the aforementioned first relationship of correspondence, it is possible to obtain the color information in the third color system corresponding to the device data in the first device space. In addition, by applying the color information in the third color system thus obtained to the aforementioned inverse relationship of correspondence, it is possible to obtain device data in the second device space corresponding the color information in the third color system. The thus-obtained device data in the second device space is made to correspond to the device data in the aforementioned first device space.
Accordingly, by applying to the inverse relationship of correspondence the color information in the third color system made to correspond to the device data in the first device space by the first relationship of correspondence, device data in the second device space corresponding to the device data in the first device space is obtained, and the forward relationship of correspondence from the deice data in the first device space to the device data in the second device space is determined.
Then, by using the forward relationship of correspondence thus determined, the device data in the first device space is transformed into the device data in the second device space.
Since the aforementioned forward relationship of correspondence represents a direct correspondence from the device data in the first device space to the device data in the second device space, by using this forward relationship of correspondence, the device data in the first device space can be transformed into the device data in the second device space without effecting a transformation which once undergoes mapping on the colorimetric space in the conventional manner.
In addition, since the device data in the first device space is transformed into the device data in the second device space by using the forward relationship of correspondence which has been determined once, conventional processing such as compression, expansion, and truncation of data in the colorimetric space is not performed during the transformation. For this reason, it is possible to prevent a decline in the color reproduction accuracy.
The color transformation method in accordance with the second aspect of the present is a color transformation method in which the RGB color system is adopted as the first color system and the CMYK color system is adopted as the second color system, and device data in the first device space represented by the RGB color system is transformed into device data in the second device space represented by the CMYK color system.
In this color transformation method, a first relationship of correspondence from the device data in the first device space to color information in a third color system and a second relationship of correspondence from the device data in the second device space to color information of the third color system are first determined.
As also described in the column for conventional art, K data has a characteristic that it is equivalently replaced by a gray component which is represented by a predetermined amount of equivalent CMY data, and innumerable CMYK data which are equivalent to one arbitrary piece of CMYK data are present, as shown in FIG. 14. Meanwhile, the CMY data has linear independence in that a color which is represented by one combination of CMY data cannot be represented by another combination of CMY data.
Accordingly, after K data of the device data in the second device space is constrained under a predetermined condition, an inverse relationship of correspondence from the color information of the third color system to the device data in the second device space is determined as an inverse solution of the second relationship of correspondence.
Here, the K data may be fixed to a preset value, or may be set in correspondence with an amount of ink necessary allowing a color corresponding to the color information of the third color system to be reproduced by a printing device. Alternatively, the K data may be determined in correspondence with the lightness or saturation of a color corresponding to the color information of the third color system. Namely, the K data is constrained under the condition that the lower the lightness and saturation of the color corresponding to the color information of the third color system, the more the K data is increased. As a result, since the K data becomes small in the case of a color having high saturation, it is possible to avoid the occurrence of muddiness when a color having high saturation is reproduced. In addition, in the case of a color whose lightness and saturation are low, since the K data becomes large, the color whose lightness and saturation are low can be reproduced stably. Hence, there is an advantage in that the K data can be constrained appropriately in correspondence with the lightness and saturation of the color corresponding to the color information of the third color system.
If the K data is constrained under the predetermined condition in the above-described manner, it is possible to narrow unknown data down to the CMY data, and it is possible to univalently obtain device data in the second device space corresponding to the color information of the third color system. Accordingly, an inverse relationship of correspondence from the color information of the third color system corresponding to the device data (RGB data) in the first device space can be determined univalently. In addition, if the second relationship of correspondence cannot be described by a simple matrix, the inverse relationship of correspondence is determined by successively obtaining inverse solutions from the second relationship of correspondence.
Next, by applying to the inverse relationship of correspondence the color information of the third color system made to correspond to the device data i the first device space by the first relationship of correspondence, device data in the second device space corresponding to the device data in the first device space is obtained, and the forward relationship of correspondence from the device data in the first device space to the device data in the second device space is determined.
Then, by using the forward relationship of correspondence thus determined, the device data in the first device space is transformed into the device data in the second device space.
Since the aforementioned forward relationship of correspondence represents a direct correspondence from the device data in the first device space to the device data in the second device space, by using this forward relationship of correspondence, the device data in the first device space can be transformed into the device data in the second device space without effecting a transformation which once undergoes mapping on the colorimetric space in the conventional manner.
In addition, since the device data in the first device space is transformed into the device data in the second device space by using the forward relationship of correspondence which has been determined once, conventional processing such as compression, expansion, and truncation of data in the colorimetric space is not performed during the transformation. For this reason, it is possible to prevent a decline in the color reproduction accuracy.
In addition, since the K data is constrained, the forward relationship of correspondence expressing a direct correspondence from three-dimensional RGB data to four-dimensional CMYK data can ca determined univalently.
As described above, in accordance with the first and second aspects of the present invention, by using the first relationship of correspondence from the device data in the first device space to the color information of the third color system and the inverse relationship of correspondence from the color information of the third color system to the device data in the second device space, the forward relationship of correspondence expressing a direct correspondence from the device data in the first device space to the device data in the second device space is determined, and the device data in the first device space is directly transformed into the device data in the second device space on the basis of the forward relationship of correspondence. Accordingly, it is possible to prevent the drawback that the color reproduction accuracy declines.
In addition, in accordance with the second aspect of the present invention, since the K data is constricted, the forward relationship of correspondence expressing a direct correspondence from the three-dimensional RGB data to the four-dimensional CMYK data can be determined univalently.
In addition, in accordance with the second aspect of the present invention, if the predetermined condition is a condition that the lower the lightness and saturation of a color corresponding to the color information of the third color system, the more the K data is increased, the K data can be constrained appropriately in correspondence with the lightness and saturation of the color corresponding to the color information of the third color system.